Easy-open lid

ABSTRACT

The present invention relates to an easy-open vessel lid which is composed of a laminate of a metal foil and a resin and has opening weakened line. A reinforcing layer of a thermoplastic resin is formed independently from the resin of the laminate at least on the outer peripheral side of a part most protruding in the opening-initiating direction of an inside introducing weakened line to cover said weakened line astride of said weakened line. By this reinforcing layer, ribbon breaking can be effectively prevented, and opening by shearing of the lid member along the weakened lines can be performed assuredly and smoothly.

DESCRIPTION

1. Technical Field

The present invention relates to an improvement in an easy-open lidcomprising a laminate of a metal foil and a resin and an openingweakened line formed in the laminate, in which occurrence of ribbonbreaking at the time of opening is prevented.

2. Technical Background

An easy-open lid comprising a laminate of a metal foil and a resin iswidely used as a sealing lid for a light-weight vessel comprising ametal foil, a resin, a paper or a laminate thereof as the constituentmaterial. In this easy-open lid, an opening weakened line defining aportion to be opened is formed in the laminate and an opening tab isfixed to the opening-initiating point of the portion to be opened, andopening is effected by breaking the lid along the weakened line.

In the field of the easy-open lid of this type, a so-called full-openlid in which an opening is formed on the substantially entire surface ofthe lid is desired. For this full-open lid, there is generally adopted astructure in which, as shown in FIG. 5-I of Japanese Patent ApplicationLaid-Open Specification No. 152145/84, a portion to be opened is definedby an outer peripheral weakened line, an outside introducing weakenedline and an inside introducing weakened line are formed, tearing iseffected in a ribbon-like shape from an opening-initiating point, andthen, the entire surface is opened.

In this full-open easy-open lid, ribbon breaking is often caused at thetime of opening, rendering opening of the entire surface difficult. Theterm ribbon breaking means the phenomenon in which the ribbon-shapedportion defined by the outside introducing weakened line and the insideintroducing weakened line undergoes a concentrated opening force at theinitial stage of opening and is broken at the point close to the partmost protruding in the opening-initiating direction of the insideintroducing weakened line. This ribbon breaking is a phenomenon notobserved in an easy-open can lid formed of an aluminum sheet, and thisis a trouble occuring inherently in a lid formed of a laminate of ametal foil and a resin.

3. Gist of the Invention

It is therefore a primary object of the present invention to provide aneasy-open lid of he above-mentioned metal foil/resin type, in whichoccurrence of ribbon breaking is prevented and opening of the entiresurface of the lid can be performed assuredly and smoothly withoutfailure.

More specifically, in accordance with the present invention, there isprovided an easy-open vessel lid comprising a lid member composed of alaminate of a metal foil and a resin, a peripheral weakened line formedin the lid member, a portion to be opened, which is defined by theperipheral weakened line, an outside introducing weakened line, which isformed in the portion to be opened and reaches the peripheral weakenedline, an inside introducing weakened line, which is formed in theportion to be opened and does not reach the outer peripheral weakenedline, an opening-initiating part and a ribbon-shaped portion, which aredefined by the outside introducing weakened line and the insideintroducing weakened line, and an opening tab connected to theopening-initiating part, wherein a reinforcing layer of a thermoplasticresin is formed independently from said resin at least on the outerperiphery side of a part most protruding in the opening-initiatingdirection of the inside introducing weakened line to cover said weakenedline astride of said weakened line.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows curves of the distribution of the tear strength of (A) alaminate having a weakened line and (B) a product obtained by forming areinforcing resin layer astride of this weakened line of this laminate.

FIG. 2 is a top view showing an embodiment of the easy-open lid of thepresent invention.

FIG. 3 is a view showing the section taken along the line III--III ofthe lid shown in FIG. 2.

FIG. 4 is a view showing the section taken along the line IV--IV of thelid shown in FIG. 2.

FIG. 5 is a top view showing a lid in which a reinforcing resin layer isformed to cover an inside weakened line and an entire extension thereof.

FIG. 6 is a view showing the section taken along the line VI--VI of thelid shown in FIG. 5.

FIG. 7 is a top view showing a lid in which a reinforcing resing layeris formed to cover all of weakened lines.

FIG. 8 is a top view showing another embodiment of the easy-open lid ofthe present invention.

FIG. 9 is a view showing the section taken along the line IX--IX of thelid shown in FIG. 8.

FIG. 10 is a view showing the section taken along the line X--X of thelid shown in FIG. 8.

FIG. 11 is a top view showing a lid similar to the lid shown in FIG. 2except that the reinforcing resin layer is not formed.

FIG. 12 is a top view showing a lid similar to the lid shown in FIG. 8except that the reinforcing resin layer is not formed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The easy-open lid of the present invention is characterized in that areinforcing layer of a thermoplastic resin is formed independently fromthe resin of the laminate at a part most protruding in theopening-initiating direction of the inside introducing weakened lineoutwardly of the opening-initiating direction to cover said weakenedline astride of said weakened line.

As pointed out hereinbefore, the reason why ribbon breaking is caused atthe time of opening is that the opening force is concentrated on a partor in its vicinity most protruding in the opening-initiating directionof the inside introducing weakened line and a cut is formed in aribbon-shaped portion defined by the inside introducing weakened lineand the outside introducing weakened line by this concentrated stress.According to the present invention, by forming a reinforcing layer of athermoplastic resin in the above-mentioned specific positional relationindependently from the resin of the laminate, occurrence of this ribbonbreaking can be effectively prevented.

In the present invention, by forming a reinforcing layer of athermoplastic resin at least on the outer periphery side of a part mostprotruding in the opening-initiating direction of the inside introducingweakened line (sometimes referred to as "most protruding part") to coversaid weakened line astride of said weakened line, occurrence of ribbonbreaking is effectively prevented and opening is effected by shearing ofthe lid along the weakened line. This was found as a phenomenon and hasnot been theoretically supported sufficiently. It is expected that if areinforcing layer of a resin is formed astride of the weakened line,shearing of not only the circumference of the weakened line but also theweakened line per se will probably become difficult. Contrary to thisexpectation, shearing along the weakened line becomes possible. Thereason is considered to be as follows.

FIG. 1 of the accompanying drawings is a conceptional view showing thedistribution of the tear strength in the portion of the weakened lineand the surrounding portion, and curve A shows the tear strengthdistribution in the laminate alone and curve B shows the tear strengthdistribution in the laminate in which the reinforcing resin layer isformed astride of the weakened line. From FIG. 1, it is seen that thefollowing relation is established between the tear strengths Tso and Tsiin the portions of weakened lines and the tear strengths To and Ti atpositions slightly apart from the weakened lines:

    Tsi-Tso<Ti-To

This relation will now be described. Even if the reinforcing resin layeris formed, since the weakened line acts as a clue to shearing, the tearstrength Tsi in the direction of the weakened line is not much largerthan the value Tso in the laminate having no reinforcing layer, but thetear strength Ti in the portion other than the weakened line is largerby the value corresponding to the increase of the thickness than thetear strength To of the laminate having no reinforcing layer. If aweakened line (score) is formed in the laminate, the thickness of theportion in the vicinity of the weakened line is reduced at theprocessing step, and the distribution of the tear strength becomesgentle as shown by curve A. In contrast, if the reinforcing layer isformed, the distribution of the tear strength abruptly rises at a partapart from the portion of the weakened line. It is considered thatbecause of the combination of these factors, shearing can be performedassuredly and easily, and ribbon breaking can be effectively prevented.

Several additional advantages can be attained in the present inventionby forming a reinforcing layer of a thermoplastic resin astride of theweakened line. In the first place, strict positioning is not necessaryat the time of applying the reinforcing layer, and the reinforcing layercan be formed assuredly on the outer periphery side of the mostprotruding portion of the inside introducing weakened line where ribbonbreaking is readily caused. In the second place, even if a defect suchas a pinhole or crack is generated in the laminate at the time offorming the weakened line, since the defect is covered with thereinforcing layer of the thermoplastic resin, complete protection can beattained.

Structure of Lid

Referring to FIGS. 2, 3 and 4 illustrating one embodiment of theeasy-open lid of the present invention, this lid member 1 is formed of alaminate comprising a metal foil 2 and a resin film 3. A peripheralweakened line 4 is formed on the lid member 1, and a portion 5 to beopened is formed inwardly of the peripheral weakened line 4 and aheat-sealing peripheral edge portion 6 is formed outwardly of theportion 5 to be opened. In the portion 5 to be opened, there are formedan outside introducing weakened line 7 on the side of the outerperiphery, which extends substantially from the center of the lid andreaches the peripheral weakened line 4, and an inside introducingweakened line 8 on the side of the center, which does not reach theperipheral weakened line 4. A ribbon-shaped part is defined by both theintroducing weakened lines 7 and 8. In this embodiment, the insideintroducing weakened line 8 has a circular extension (weakened line) 10concentric with the peripheral weakened line 4, which has a diametersmaller than that of the peripheral weakened line 4.

An opening-initiating part 11 is formed at the beginning ends of boththe introducing weakened lines 7 and 8. In the present embodiment, theopening-initiating part 11 is a V-shaped small hole, and an opening tab12 is formed to cover the V-shaped small hole from the outer side, andan inner patch 13 is formed to cover the V-shaped small hole from theinner surface side. The small hole 11 is sealed by fusion-bonding theopening tab 12 and inner patch 13 to each other. A grip 14 is formed atthe end of the opening tab 12. The center line of the opening tab 12 isordinarily made substantially in agreement with the center line of theribbon-shaped part 9, and this direction, that is, the direction ofarrow A in FIG. 2, is the opening-initiating direction.

Referring to FIG. 2, the portion most protruding in theopening-initiating direction of the inside introducing weakened line 8is located at a position indicated by B, and at the time of opening, acut is formed in the ribbon-shaped portion 9 from this position B tocause ribbon breaking. In the present invention, a reinforcing layer 15of a thermoplastic resin is formed to cover the periphery of theweakened line 8 astride of the weakened line 8 at the position B and inthe vicinity thereof. In the embodiment illustrated in FIGS. 2 through4, the inner patch 13 and reinforcing resin layer 15 are integrallyformed of one film, and as clearly shown in FIG. 2, one ellipsoidal filmcovering both the portions is applied to the inner surface of the lid.Of course, it will be understood that the inner patch 13 and reinforcingresin layer 15 can also be formed of two individual films.

When the lid of the present invention is opened, by gripping the grip 14of the opening tab 12 and pulling it upward, the inner patch 13 israised in the portion of the hole 11 in the state adhering to theopening tab 12 to initiate the opening. Then, both the introducingweakened lines 7 and 8 are shown and the ribbon-shaped part 9 is raised.In this case, for the above-mentioned reason, ribbon breaking isprevented also at the most protruding part B of the inside introducingweakened line 8. The opening force of pulling the opening tab 12 isconverted to a turning force, and shearing of the peripheral weakenedline 4 and extended weakened line 10 is advanced and full openingbecomes possible.

In the present invention, the reinforcing resin layer 15 can be formedat an optional position with an optional shape without departing fromthe spirit of the present invention. For example, as shown in FIGS. 5and 6, the reinforcing resin layer 15 is formed to entirely cover theinside introducing weakened line 8 and the extended weakened line 10thereof, or as shown in FIG. 7, the reinforcing resin layer 15 is formedto cover all of the weakened lines 4, 7, 8 and 10.

In the easy-open lid of the present invention, the opening tab can befixed to the lid member by riveting. Referring to FIGS. 8, 9 and 10illustrating this embodiment, beginning ends of the outside introducingweakened line 7 and the inside introducing weakened line 8 are connectedto each other to form a portion 16 having a large curvature. A smallhole 11' is formed on the side of the center of the large curvatureportion. A rivet 17 of the opening tab 12 is inserted in this small hole11' and the end portion of the rivet 17 is expanded, and the rivet 17 isfusion-bonded to the resin layer 3 of the lid 1 to effect fixation andsealing of the opening tab 12. The reinforcing resin layer 15 is formedon the outer surface of the lid 1 to cover the most protruding part ofthe inside introducing weakened line and the surrounding portionthereof.

Constituents of Lid and Process for Preparation of Lid

A light metal foil such as an aluminum foil is preferably used as themetal foil of the laminate. Of course, an iron foil, a steel foil and atinplate foil can be used. In view of the resistance to the heatingsterilization, it is preferred that the metal foil be one that has beensubjected to a preliminary treatment such as an alumite treatment, aboehmite treatment, a chemical treatment with phosphoric acid and/orchromic acid or a conversion treatment.

The metal foil should have a certain rigidity enough to make tearingpossible, and from this viewpoint, it is preferred that the thickness ofthe metal foil be at least 30 μm, especially at least 50 μm. From theeconomical viewpoint and in order to prevent the finger or the like frombeing hurt at the time of opening, it is preferred that the upper limitof the thickness of the metal foil be 200 μm, especially 150 μm.

As the resin of the laminate, resins having a heat sealability, forexample, olefin resins such as low-density polyethylene, medium-densitypolyethylene, high-density polyethylene, isotactic polypropylene, apropylene/ethylene copolymer, a propylene/butene-1 copolymer, anethylene/vinyl acetate copolymer (EVA), an ethylene/ethyl acrylatecopolymer and an ionomer, are used. It is preferred that the thicknessof the resin layer be 5 to 100 μm, especially 10 to 70 μm. Lamination ofthe resin layer and metal foil can be performed by using, for example, aurethane type adhesive, an epoxy type adhesive and an acid-modifiedolefin type adhesive.

The laminate has a multi-layer structure including other layers inaddition to the above-mentioned two layers. For example, a paper layeror protecting resin layer can be formed on the surface of the metalfoil. As the protecting resin layer, there can be used a high-tenacityplastic film such as a biaxially drawn polyester film, a biaxially drawnpolypropylene film or a biaxially drawn nylon film, and a coating of anepoxy-phenolic paint, an epoxy-urea paint, an epoxy-melamine paint, avinyl paint, an acrylic paint or an epoxy-acrylic paint.

The laminate is formed into a lid through press forming (draw forming),punching, formation of a surrounding curled portion and draw forming ofa panel portion. Then, scores are formed in the metal foil and holes arepierced.

Alternately, there can be adopted a method in which holes and scores areformed in the laminate, the opening tab and reinforcing layer arebonded, and the laminate is punched to form a lid.

In view of the gas barrier property, the pressure resistance and theresistance to the falling shock, it is important that scores shouldremain in the midway of the thickness of the metal foil. In view of theabove-mentioned characteristics and the easy openability, it ispreferred that the depth of the scores be 3/10 to 7/10, especially 2/5to 3/5, of the thickness of the metal foil and the residual thickness ofthe metal foil in the scored portion be at least 10 μm, especially atleast 20 μm.

As the opening tab, there can be used a tab formed from a laminatehaving a layer structure similar to the layer structure mentioned abovewith respect to the lid member, and an opening tab provided with a rivetformed of an olefin resin similar to the inner face material of the lidmember.

Various resin films can be used as the reinforcing layer of thethermoplastic resin, but use of a film of an olefin resin similar to theinner face material of the lid member is especially advantageous. It ispreferred that the thickness of the reinforcing layer be 20 to 150 μm,especially 40 to 100 μm, and the tensile modulus be 20 to 150 kg/mm².

Fixation of the opening tab or bonding of the reinforcing layer to thelid can be advantageously performed by the heat bonding method such asheat fusion bonding, but needless to say, bonding and fixation can beaccomplished by using an adhesive or the like.

The lid of the present invention is advantageously used as aheat-sealing lid for heat-sealing an optional vessel, for example, apaper/plastic/aluminum foil composite vessel, a metal can, a wide-mouthbottle, a plastic cup, a metal foil vessel or a metal foil/plasticcomposite vessel. Particularly, the lid of the present invention isadvantageously used for sealing an easy-buckling vessel to which doubleseaming cannot be applied, and a packaged vessel requiring hot filling,heat sterilization, and the lid of the present invention is especiallypreferably used as a heat-sealing lid for a plastic cup prepared byvacuum forming, a monoaxially or biaxially drawn plastic cup obtained byplug assist forming or compressed air forming, or a metal foil vesselobtained by draw forming. As typical instances of the vessel to whichthe lid of the present invention is applied, there can be mentioned avessel having an aluminum or steel foil and a heat-sealable resin filmlayer formed on the inner and outer surfaces of the foil, and acomposite can or cup-shaped vessel provided with a paper and a metalfoil.

According to the present invention, by forming a reinforcing layer of athermoplastic resin at least on the outer peripheral side of the portionmost protruding in the opening-initiating direction of the insideintroducing weakened line to cover said weakened line astride of saidweakened line, ribbon breaking can be effectively prevented, and openingby shearing of the lid along the weakened line can be performedassuredly and smoothly. Furthermore, by forming the reinforcing layer ofthe thermoplastic resin astride of the weakened line, strict positioningis not necessary when the reinforcing layer is applied, and thereinforcing layer can be formed assuredly on the outer peripheral sideof the most protruding portion of the inside introducing weakened linewhere ribbon breaking is readily caused. Moreover, even if a defect suchas a pinhole or crack is generated on the laminate when a weakened lineis formed, this defect is covered with the reinforcing layer of thethermoplastic resin and complete protection can be obtained. These areadvantages attained by the present invention.

EXAMPLES EXAMPLE 1

A soft aluminum foil having a thickness of 50 μm, on the outer surfaceof which an epoxy-phenolic paint had been coated and baked, wasprepared, and a polypropylene film having a thickness of 65 μm wasextrusion-laminated on the inner surface of the aluminum foil to obtaina lid material.

The lid material was punched into a disk having a diameter of 80 mm bypress forming, and a V-shaped small hole was formed through the disk inthe central portion thereof by press forming. A weakened line was formedon the coated and baked side of the lid member by press forming so thatthe weakened line was connected to the top end of the V-shaped smallhole and the residual thickness in the portion of the weakened line wasabout 25 μm, whereby a lid member having a shape as shown in FIGS. 2, 3and 4 was prepared.

A biaxially drawn polyester film having a thickness of 12 μm was bondedto one surface of a soft aluminum foil having a thickness of 9 μm byusing an adhesive, and a polypropylene film having a thickness of 50 μmwas extrusion-laminated on the other surface of the aluminum foil. Anopening tab having a width of 20 mm and a length of 35 mm was cut outfrom the obtained laminate. A reinforcer having a width of 20 mm and alength of 33 mm was formed from a polypropylene film having a thicknessof 50 μm. The opening tab was placed on the outer surface of the lidmember in such a manner that the V-shaped small hole of the lid memberwas covered with the opening tab, and simultaneously, the reinforcer wasplaced on the inner surface of the lid member to cover said small holeand the surrounding portion of the part most protruding in theopening-initiating direction of the inside introducing weakened line.The reinforcer and a heat-resistant rubber sheet having a thickness of 1mm were placed face to face and press-bonded and the opening tab and lidmember were placed face to face and pressed by a hot plate heated at200° C. in such a manner that a peelable seal was produced between theouter surface of the lid member and a tight seal was produced in theinner surface of the lid member and the opening tab to the reinforcer,whereby heat sealing was completed. By this heat sealing, the openingtab was connected to the opening-initiating part and an easy-open vessellid of the present invention, as shown in FIGS. 2 through 4, wasprepared.

Corn soup was hot-filled in a cylindrical cupshaped vessel having aninner volume of about 80 cc, which was prepared by deep draw forming ofa laminate obtained by extrusion-laminating polypropylene on the innerand outer surfaces of a steel foil having a thickness of 75 μm, and theabove-mentioned easy-open vessel lid of the present invention washeat-sealed to the vessel. The lidded vessel was subjected to the retortsterilization at 115° C. for 50 minutes. Then, the opening tab was heldby the hand and the opening was effected in the opening-initiatingdirection. The result of the evaluation of the openability (frequency ofoccurrence of ribbon breaking) is shown in Table 1.

EXAMPLE 2

An easy-open vessel lid as shown in FIGS. 5 and 6 was prepared by usingthe same materials according to the same procedures as described inExample 1. The reinforcer of polypropylene was arranged on the innersurface of the lid member to entirely cover the small hole of the lidmember, the inside introducing weakened line and the extension thereof.The openability was evaluated according to the same procedures asdescribed in Example 1. The obtained result is shown in Table 1.

EXAMPLE 3

A disk having a diameter of 80 mm was punched out from the same lidmaterial as described in Example 1 by press forming, and a circularsmall hole having a diameter of 4 mm was formed at the center of thedisk by press forming. Weakened lines were formed on the outer surfaceside of the disk in the same manner as described in Example 1 exceptthat the weakened line in the vicinity of the circular small hole wasformed, whereby a lid member as shown in FIG. 8 was prepared. Areinforcer having a diameter of 20 mm was prepared from a polypropylenefilm having a thickness of 50 μm. The reinforcer was placed on the outersurface of the lid member to cover the surrounding portion of the partmost protruding in the opening-initiating direction of the insideintroducing weakened line. The inner surface of the lid member and aheat-resistant rubber sheet having a thickness of 1 mm were placed faceto face and pressbonded and the reinforcer was pressed by a hot plateheated at 200° C., whereby heat sealing was completed. An opening tab ofpolypropylene having a cylindrical rivet having a diameter of 3.9 mm anda length of 4 mm, which could be fitted in the above-mentioned smallhole, and a ring in which a forefinger could be inserted, was preparedby injection molding. The rivet of the opening tab was fitted from theouter surface side of the lid member, and the head portion protruding tothe inner surface side of the lid member was crushed by a hot plateheated at 200° C. and fusion-bonded to the polypropylene of the innersurface, whereby an easy-open vessel lid of the present invention, asshown in FIG. 8, was prepared.

The openability was evaluated according to the same procedures asdescribed in Example 1. The obtained result is shown in Table 1.

COMPARATIVE EXAMPLE 1

An easy-open vessel lid as shown in FIG. 11 was prepared by using thesame materials according to the same procedures as described inExample 1. The inner patch was arranged to cover the small hole but didnot cover the part most protruding in the opening-initiating directionof the inside introducing weakened line.

The openability was evaluated according to the same procedures asdescribed in Example 1. The obtained result is shown in Table 1.

Comparative Example 2

A disk having a diameter of 80 mm was punched out from the same lidmaterial as used in Example 1 by press forming, and a circular smallhole having a diameter of 4 mm was formed at the center of the disk bypress forming. Weakened lines were formed on the outer surface side ofthe disk in the same manner as described in Example 1 except that theweakened line in the vicinity of the circular small hole was formed,whereby a lid member as shown in FIG. 12 was prepared. An opening tabhaving a cylindrical rivet having a diameter of 3.9 mm and a length of 4mm, which could be fitted in the circular small hole, and a ring inwhich a forefinger could be inserted, was prepared by injection molding,and the rivet of the opening tab was fitted from the outer surface sideof the lid member and the head of the rivet protruding to the innersurface side of the lid member was crushed by a hot plate heated at 200°C. and fusion-bonded to the polypropylene of the inner surface, wherebyan easy-open vessel lid as shown in FIG. 12 was prepared. Incidentally,the reinforcer was not bonded to this vessel lid. The openability wasevaluated in the same manner as described in Example 1. The obtainedresult is shown in Table 1.

From the results shown in Table 1, it is seen that occurrence of ribbonbreaking is much controlled in the lids of the present invention ascompared with the comparative lids.

                  TABLE 1                                                         ______________________________________                                                                       Compar-                                                                              Compar-                                                                ative  ative                                   Example      Example  Example  Example                                                                              Example                                 1            2        3        1      2                                       ______________________________________                                        Frequency                                                                             0/15     0/15     0/15   14/15  13/15                                 of Occur-                                                                     rence of                                                                      Ribbon                                                                        Breaking                                                                      (lids/lids)                                                                   ______________________________________                                    

What is claimed is:
 1. An easy-open vessel lid formed of a laminatecomprising a metal foil and a resin layer, said lid comprisingaperipheral weakened line formed in the lid; a portion to be openedformed inwardly of the peripheral weakened line; a heat-sealingperipheral edge portion formed outwardly of the peripheral weakenedline; an opening-initiating weakened line having a large curvatureprotruding in an opposite direction to an opening initiating directionformed in the portion to be opened; an outside introducing weakened,arched line, one end of which is connected to one end of theopening-initiating weakened line, the other end of which reaches theperipheral weakened line, and which extends in the opening-direction; aninside introducing weakened circumferential line, one end of which isconnected to the other end of the opening-initiating weakened line andwhich has an introducing part extending in the opening-initiatingdirection, a most protruding part in the opening-initiating directionand a circular extension substantially concentric with the peripheralweakened line, said extension having a diameter smaller than that of theperipheral weakened line; an opening-initiating part defined by theopening-initiating weakened line; a spiral ribbon-shaped portionconnected to said opening-initiating part which is defined by saidoutside introducing weakened, arched line and said inside introducingweakened circumferential line; an opening tab fitted to the outersurface of the opening-initiating part; and a reinforcing layer of athermoplastic resin adhered to said laminate so as to cover the mostprotruding part of the inside introducing weakened circumferential lineastride said inside introducing weakened circumferential line.
 2. Aneasy-open vessel lid as set forth in claim 1, wherein the reinforcinglayer of the thermoplastic resin is arranged at a position entirelycovering the inside introducing weakened line and the extension thereof.3. An easy-open vessel lid as set forth in claim 1, wherein thereinforcing layer of the thermoplastic resin is arranged to cover all ofthe weakened lines.
 4. An easy-open vessel lid as set forth in claim 1,wherein the reinforcing layer is an olefin resin film.
 5. An easy-openvessel lid as set forth in claim 1, wherein the thickness of thereinforcing layer is 20 to 150 μm.
 6. An easy-open vessel lid as setforth in claim 1, wherein the tensile modulus of the reinforcing layeris 20 to 150 kg/mm².
 7. An easy-open vessel lid formed of a laminatecomprising a metal foil and a resin layer, said lid comprisingaperipheral weakened line formed in the lid; a portion to be openedformed inwardly of the peripheral weakened line; a heat-sealingperipheral edge portion formed outwardly of the peripheral weakenedline; an opening-initiating V-shaped hole having a large curvatureprotruding in an opposite direction to an opening-initiating directionformed in the portion to be opened; an outside introducing weakened,arched line, one end of which is connected to one end of the V-shapedhole, the other end of which reaches the peripheral weakened line, andwhich extends in the opening-direction; an inside introducing weakenedcircumferential line, one end of which is connected to the other end ofthe V-shaped hole and which has an introducing part extending in theopening-initiating direction, a most protruding part in theopening-initiating direction and a circular extension substantiallyconcentric with the peripheral weakened line, said extension having adiameter smaller than that of the peripheral weakened line; a spiralribbon-shaped portion defined by said outside introducing weakened,arched line, said inside introducing weakened circumferential line andsaid V-shaped hole; an opening tab fitted to the outer-surface of thelid so as to cover the V-shaped hole; a reinforcing layer of athermoplastic resin bonded to the inner surface of said lid so as tocover the most protruding part of the inside introducing weakenedcircumferential line astride said inside introducing weakenedcircumferential line, and to cover the V-shaped hole; and anopening-initiating part formed by bonding said opening tab with saidreinforcing layer through said V-shaped hole and sealing said V-shapedhole.